Low-temperature behavior of natural kalsilite with P31c symmetry: An in situ single-crystal X-ray diffraction study

Abstract

The low-temperature behavior of a natural kalsilite (ideal formula KAlSiO₄) with P31c symmetry has been investigated by in situ single-crystal diffraction. A series of intensity data collections and structural refinements have been performed at 298, 250, 200, 150, and 100 K on decreasing temperature, and 175, 225, and 275 K on increasing T. The variations of the unit-cell parameters of kalsilite as a function of T are continuous, and show no evidence of any phase transitions or thermo-elastic anomalies in this temperature range. An expansion is observed along [0001] with decreasing temperature. The axial and volume thermal expansion coefficients (α_j = l_j^-1⋅∂l_j/∂T, α_V = V^-1⋅∂V/∂T) between 298 and 100 K, calculated by weighted linear regression through the data points, are α_a = α_b = 1.30(6)·10^-5, α_c = -1.5(1)·10^-5, α_V = 1.1(2)·10^-5 K^-1. The main structural change on decreasing temperature is a cooperative anti-rotation of tetrahedra forming the six-membered rings lying parallel to (0001). This tetrahedral rotation is coupled with a change in the distances between the extra-framework cations and the framework O atoms. A small decrease in the tetrahedral tilts perpendicular to [0001] is responsible for the negative thermal expansion along [0001]; the implications of these mechanisms for thermal expansion in nephelines and kalsilites are discussed